Energy consumption of lighting systems is presently a major societal concern. This concern has even led to some governments to encourage the development of more energy efficient light sources and smart controls for those light sources. Some methods of improving light efficiency rely on improvements of the underlying materials and structures of the lighting devices themselves. Other methods involve designing customized spectral distributions that satisfy color quality and luminous efficacy. Still other methods require designing light management systems with sophisticated sensors and algorithms to control the intensity of lights depending on ambient light, occupancy, and usage.
For the specific case of multi-layered surface light emitting devices/diodes, including inorganic light emitting diodes (LEDs) and organic light emitting devices (OLEDs), only a small fraction of the generated light often leaves the device. This inefficiency is because much of the generated light becomes trapped in the device, e.g., due to total internal reflection at a layer interfaces as well as at output surface, in addition to absorption of the light by the metal electrode. For example, for a typical OLED comprised of several organic material layers sandwiched by two electrode layers, anode and cathode, only about 20% of the light may be directly emitted into the air. The rest of the light is either trapped inside the substrate (as a substrate mode), inside the organic layers (as a waveguide mode), or absorbed by the metal cathode (usually called a plasmon mode).
Among these many light loss channels, the plasmon loss (may also be termed “plasmonic loss”) may amount to about 40% of the total emitted light and thus, the elimination of plasmon loss represents an ongoing challenge in improving the light extraction of (O)LEDs. It is thought that plasmon loss occurs mainly through the coupling of the emitted light into surface plasmon polariton (SPP) waves. SPP waves are surface waves confined to propagating along the interface between the metal electrode layer and the emitting material layer. Energy in the SPP waves is eventually absorbed by the metal electrode and thus leads to inefficient operation of the (O)LED.
Plasmon loss in light emitting devices may be reduced by using a periodic grating-structured electrode having a period that is comparable to the wavelength of the emitted light so as to induce Bragg scattering of the SPP wave into free light. However, existing grating structures are only effective for plasmon loss reduction in a very narrow wavelength range (i.e., monochromatic sources) and none are effective for broadband, i.e., white light, sources such as white OLEDs.